This invention relates generally to injection-molded parts incorporating inserts and, more particularly, to methods for pre-conditioning such inserts for improved bonding to the injection-molded material and for improved replication of complex shapes.
It is often desirable to enhance or alter the properties of an injection-molded plastic part. One method to accomplish this is to include in the mold an insert having an additional desired feature. For instance, an insert having exceptional abrasion resistance can be placed in a mold, and softer plastic injected behind it, to create a composite part having a scratch-resistant outer surface. Similarly, an insert having specific optical properties or decorations can be used, to impart a specific optical performance or stylized appearance to the subsequently molded part.
However, challenges can arise when the insert and the molding plastic are formed of dissimilar materials, or when the resultant composite part must very accurately replicate the molding shape. These challenges generally increase with the complexity of the molding shape.
For example, in the case of an optical part, such as a lens, the part must exhibit excellent replication of the molding surface to perform its function. However, if this molding surface has a significantly different curvature from that of the insert placed against it, the insert might not fit or replicate this surface properly, resulting in optical distortion or incorrect optical power. In addition, the mismatch in shapes might irreparably damage the insert, e.g., by wrinkling, buckling, or burning from inconsistent thermal contact, causing optical and cosmetic defects in the molded part.
Various methods to join single sheets of one material to a molten plastic of a second material during injection molding have been described, for example, in U.S. Pat. Nos. 5,084,226, 6,432,327, and 6,554,421. However, with thicker, multi-layer, or more resilient inserts, the insert might not conform to the mold without further steps to encourage replication. One common approach is to pre-shape the multi-layer insert into contours similar to that of the final part, using heat and pressure or vacuum force, as described, for example, in U.S. Pat. Nos. 5,599,608 and 6,177,032, and in published U.S. Patent Application No. 2002/0080488 and in European Patent No. EP 0299 509 B1, all of which are incorporated herein by reference. This approach is sufficient in many instances to allow the shape of the composite part to replicate that of the mold. However, for more complex shapes, it is more difficult to pre-form the wafers with sufficient accuracy that they do not deform in use and thus add to cosmetic or optical distortions and aberrations. This is especially difficult when attempting to replicate highly asymmetrical or structured shapes. The problem is further compounded for optical parts, which must accurately control light paths to focus, magnify, or spectrally alter the light.
In addition to shape mismatches leading to poor replication, insert molding also can be complicated by poor bonding between the insert and the injected plastic. Again, this can burn or wrinkle the insert, and it can cause spot delaminations that can degrade the part's optical, cosmetic and structural integrity. A general weakness of the bond also can bring about unacceptable failures during normal use of the part. Improved bonding between discrete parts can be provided by appropriate surface treatments, by the use of adhesives, or by the use of local: heating, as described in U.S. Pat. No. 5,853,510. However, these approaches might not be suitable or effective for injection molding, because the insert will be subjected to much more extreme conditions of heat and pressure during the molding process, which can mitigate any pretreatment. Consequently, alternative methods to strengthen this bond are indicated.
It should, therefore, be appreciated that there remains a need for an improved method for bonding an insert to a substrate during an injection molding process, which is not unduly time-consuming, and which is more adaptable to a broad range of molded parts. The present invention satisfies this need.